This invention discloses mixtures for use as binders for solvent-free lacquers which are liquid at room temperature, can be prepared in the presence of moisture, and have good light stability and resistance to weather.
Paints which are applied by the usual methods such as spraying, brush coating, dipping, casting or application with rollers are normally mixed with solvents and diluents to adjust them to the viscosity most suitable for the given process of application. These solvents and diluents evaporate after application of the paint. The solvents must be recovered and burnt or otherwise removed because they constitute a nuisance to the worker, safety risk and an ecological problem. Attempts have therefore already been made to replace these solvents by water but watersoluble or water miscible coating compounds produced for these purposes in most cases contain alcohols or glycols and sometimes amines which give rise to problems of waste water pollution. Moreover it has not so far been possible to produce so-called dispersion paints which satisfactorily combine the desired properties of gloss, permanence and mechanical strength.
Although dispersions of organic polymers (so-called non-aqueous dispersions) in certain solvents which are less harmful from the point of view of industrial hygiene can be produced with solids contents of 60% or more, coats produced from such dispersions are still unsatisfactory in their qualities and the flash point of the solvents used is still too low.
So-called powder lacquers have therefore increasingly come into use in recent times. These are dry, pigmented lacquer powders with particles mostly measuring less than 100.mu. which are applied to a workpiece in an electrostatic field and then melted at an elevated temperature, the melting process being in some cases accompanied by a crosslinking reaction. This process has, however, the disadvantages of requiring the use of extruders, which are apparatus not normally used in the lacquer industry, and also that it is very difficult to produce a given color shade and the change of shade is very time consuming for the user.
Attempts were therefore made at a very early stage in the development of this process, especially in the case of so-called two-component reaction systems, to produce solvent-free low viscosity systems either by selecting suitable reaction components or by adding diluents which were reactive and could therefore be incorporated in the systems. Examples of substances used in such systems include unsaturated polyesters dissolved in monostyrene and catalyzed with peroxides, solutions of oligomeric acrylates in monoacrylic acid esters or methacrylic acid esters, solutions of polyepoxides in epichlorohydrin, etc. Combinations of relatively low viscosity aromatic diisocyanates or polyisocyanates and hydroxyl polyethers which can be diluted with reactive diluents such as phenoxyethanol are also known.
All the known processes outlined above for producing solvent-free paints have various disadvantages which may in some cases be serious. None of these systems is capable of combining all the properties required, for example, by decorative paints used on motor cars, refrigerators, furniture, machinery, etc. In some cases the capacity for taking up pigments in unsatisfactory and, in others, the surface adherence or mechanical strengths and, in most cases, the lightfastness is insufficient or the pot life or spread coating properties may be quite inadequate.
Some progress has been made with products described in German Offenlegungsschrift No. 2,006,055 which consist of combinations of polyhydroxypolyethers, monoalcohols and polyisocyanates, especially aromatic polyisocyanates, which can be applied by electrostatic spray processes. These products, however, have the serious disadvantage of being insufficiently lightfast and weather resistant for many purposes even if the preferred aromatic polyisocyanates are replaced by aliphatic polyisocyanates. A further disadvantage is that the reaction mixtures used must be prepared with the most rigorous exclusion of moisture, so that all the raw materials used must generally be subjected to a special drying operation.
It is therefore an object of this invention to provide binders for solvent-free lacquers which are liquid at room temperature in which the disadvantages mentioned above would be obviated.
According to the invention, this problem could be solved by using as binders certain mixtures of low viscosity polyisocyanates in which all the isocyanate groups are aliphatically bound, certain polyhydroxyl compounds which are free from ether groups and which may also contain certain reactive diluents.